1. Field of the Invention
The present invention generally relates to a technical field of radio communication, and, in particular, to a receiver and a receiving method for separating signals transmitted from a plurality of transmission antennas into the respective ones.
2. Description of the Related Art
In such a type of technology, in view of increasing a communication capacity, a radio communication technology of a multi-input multi-output (MIMO) method has attracted attention. In this technology, a plurality of antennas are provided in each of a transmission side and a reception side, channels created between the respective antennas are used, and thus, a communication capacity is increased (see non-patent document 1, mentioned below, for example for the MIMO method). Further, in view of increasing frequency usage efficiency in addition to bearing force against a multi-path propagation environment, a radio communication technology of an orthogonal frequency division multiplexing (OFDM) method has attracted attention. In the OFDM method, signals are transmitted with the use of a plurality of mutually orthogonal sub-carriers disposed on a frequency axis, and thus, frequency selective fading or an influence of multi-path environment are suppressed. Further, a radio communication system in which the MIMO method and the OFDM method are combined has been taken a hopeful view (see non-patent document 2, mentioned below, for such a system).
FIG. 1 shows a general outline of the MIMO method. As shows, N antennas are provided on a transmission side, transmitted signals x0 through xNt-1 are transmitted from the respective antennas. These transmitted signals are transmitted at the same time at the same frequency, but distances and disposing manners among these respective transmission antennas are appropriately set so that these can be transmitted independently. The transmitted signals transmitted from the respective antennas are received by Nr (≧Nt) reception antennas, and thus, Nr received signals y0 through yNr-1 are obtained. In the figure, signals n0 through nNr-1 added to the respective received signals show noise components, respectively. Radio sections between the transmission antennas and the reception antennas are represented by a channel matrix H, and each matrix element Hnm corresponds to a channel transfer function between the m-th transmission antenna and the n-th reception antenna. In the example of the figure, 0≦m≦Nt−1 and 0≦n≦Nr−1.
FIG. 2 shows a general outline of a transmitter in a common OFDM method. A transmitted signal, after being modulated, mapped in a predetermined signal point, undergoes serial to parallel conversion (S/P 202), undergoes inverse fast Fourier transform (IFFT 204), and thus, modulation according to the OFDM is carried out. Signals in a time domain after IFFT undergo parallel to serial conversion (P/S 206), guard intervals are added thereto (GI 208), and after that, the signals are transmitted from transmission antennas 210. It is noted that, as a mapping method of the signals, QPSK, 16QAM, 64QAM or other arbitrary method, may be adopted.
FIG. 3 shows a general outline of a receiver in a common OFDM method. The guard intervals of signals received by reception antennas 302 are removed (-GI 306). After that, the received signals undergo serial to parallel conversion (S/P 306), and undergo fast Fourier transform (FFT 308). Thereby, demodulation according to the OFDM method is carried out. The signals in a frequency domain after the transform undergo parallel to serial conversion (P/S 310), then undergo demodulation (312), and undergo other processing such as decoding.
FIG. 4 shows a general outline of a transmitter used in a system combining the MIMO method and the OFDM method. As shown, Nt transmitted signals are separated into Nt signals by means of serial to parallel conversion (S/P 402). The respective Nt signals separately undergo signal processing, and then, are transmitted from Nt transmission antennas separately. For example, a first transmitted signal is encoded (404-1), mapped (406-1), undergoes inverse fast Fourier transform (408-1), then, a guard interval is added thereto (410-1), and the signal is transmitted from a transmission antenna 412-1. the other transmitted signals are processed in the same manner, and thus, are transmitted.
FIG. 5 shows a general outline of a receiver used in the system combining the MIMO method and the OFDM method. As shown, received signals are received by Nr reception antennas 502-1 through Nr, guard intervals are removed therefrom (504-1 through Nr), and separately undergo fast Fourier transform (506-1 through Nr). The signals after undergoing the Fourier transform are separated into Nt transmitted signals (508), and demodulation and decoding are carried out on each of these transmitted signals.
For the signal processing in the signal separation part 508, there are various methods for separating the respective transmitted signals transmitted from the plurality of transmission antennas, from the signals received by the plurality reception antennas. A first method utilizes an algorithm called a zero forcing method. In this method, a pseudo inverse matrix H+ of a channel matrix H is calculated, the received signal is multiplied by the pseudo inverse matrix, and thus, the transmitted signal is obtained.
A second method utilizes an algorithm called a minimum mean squire error (MMSE) method. In this method, the received signal is multiplied by a matrix expressed by (αI+H*H)−1H*, and thus, the transmitted signal is obtained. There, a denotes α reciprocal of a signal to noise ratio (SNR−1), I denotes a unit matrix, and H* denotes a conjugate transposed matrix of the matrix H.
A third method utilizes an algorithm called a zero forcing BLAST (ZF-BLAST: Zero Forcing Bell Laboratories Layered Space Time) method. In this method, separation and removal of the signal from the transmission antenna are carried out repetitively, and thus, high-speed data transmission is achieved (for this method, see a non-patent document 3, mentioned below).
A fourth method utilizes an algorithm called a minimum mean square error BLAST (MMSE BlAST: Minimum Mean Squire BLAST) method. In this method, the minimum mean square error method and the BLAST method are combined.
A fifth method utilizes an algorithm called a maximum likelihood decoding (MLD) method. In this method, squire Euclidean distances between combinations of all the possible transmitted symbols and the received signals, and a combination providing a minimum distance is determined as a most likelihood transmitted signal.    [Non-Patent Document 1] A. Van Zelst, “Space division multiplexing algorithm”, Proc. 10th Med. Electrotechnical Conference 2000, pp. 1218-1221;    [Non-Patent Document 2] A. Van Zelst et al., “Implementation of a MIMO OFDM based wireless LAN system”, IEEE Trans. Signal. Process. 52, no. 2, 2004, pp. 483-494; and    [Non-Patent Document 3] P. W. Wolniansky et al., “V-BLAST: An architecture for realizing very high data rates over the rich scattering wireless channel”, in Proc. Int. Symposium on Advanced Radio Technologies, Boulder, Colo., September 1998.